Start your tractor up, let it run til it stops and imediately check for spark. If you don"t have one, or if it"s yelloweidh or reddish blue,then the ignition system is your problem. the spark has to e FAT AND BLUISH-WHITE, the color of lightning.
If you have a 12v electrical system, do you have a coil with ~3.2 ohms primary resistance? If you have a ~1.6 ohm coil on a 12V system, you maybe overheating the coil and causing it to internally short.
Do you have adequate fuel flow to the carb? Remove the plug from the bottom of the fuel bowl, with the tank valve wide open. You should have a CONTINUOUS gush of fuel. If it dribbles or is intermittent, you have a restriction in the fuel delivery system. Start at the fuel inlet elbow on the carb, checking the screen there. the check the flter in the sediment bowl. Then check the inlets to the tank valve and finally make sure the the cap vent is clear. If you don"t find the problem there, you"ll have to remove the carb and check the float valve and the flaot pasae for blockage.
Do these checks systematically and check by starting and running anytime you find some thing wrong and fix it.